1. Field of the Invention
The present invention relates generally to a gas turbine engine, and more specifically to a turbine stator vane with an endwall mate face seal and cooling design.
2. Description of the Related Art Including Information Disclosed Under 37 CFR 1.97 and 1.98
A gas turbine engine includes a turbine having one or more stages or rows of stator vanes and rotor blades in which a hot gas flow is passed through the convert the energy from the hot gas flow into mechanical work to drive a compressor and, in the case of an industrial gas turbine (IGT) engine, an electric generator. The first stage stator vane is exposed to the highest gas flow temperature, since the first stage is exposed to the gas flow directly from the combustor outlet.
The efficiency of the engine can be increased by passing a higher temperature gas flow into the turbine. However, the turbine inlet temperature is limited to the material properties of the turbine and an amount of cooling used in the airfoils, especially the first stage airfoils. Not only is adequate cooling required, but certain areas of the airfoils and platforms or endwalls must be kept below specific metal temperatures so that erosion damage will not occur. The hot gas flow is not at a consistent temperature throughout. Also, the hot gas flow can migrate to areas around the airfoils and into cavities outside of the normal hot gas flow path. These instances can create hot spots on certain sections of the blades or vanes. Hot spots will cause erosion damage in which the metal material will erode away and leave the surface weakened or open a hole in which the hot gas can ingest into the inside passages within the airfoil.
In a stator vane as seen in FIG. 1, the airfoil extends between an inner endwall and an outer endwall. A bow wave driven hot gas flow ingestion affect is created when the hot gas core flow entering the vane row where the leading edge of the vane induces a local blockage and therefore creates a circumferential pressure variation at an intersection of the airfoil leading edge location. The leading edge of the turbine vane generates an upstream pressure variation which can lead to hot gas ingress into a front gap (see the curved arrow in the leading edge of the inner endwall in FIG. 1). A high pressure ahead of the vane leading edge is greater than the pressure inside the cavity formed below the inner endwall. This leads to radial inward flow of the hot gas into the cavity. The ingested hot gas flows through a gap between adjacent endwalls circumferentially inside the cavity towards a lower pressure zone, and finally outflow of the hot gas at the points where the cavity pressure is higher than the local hot gas flow pressure. FIG. 2 shows a top view of a pair of adjacent vanes where the hot gas ingestion flows into the vane mate face gap. If proper cooling or design measures are not undertaken to prevent this hot gas ingress, the hot gas ingress can lead to severe damage to the front edge of the vane endwall as well as the sealing material or mate face in-between the vane endwalls of adjacent vanes.
In general, the size of the bow wave is a strong function of the vane leading edge diameter and distance of the vane leading edge to the endwall edge. The pressure variation in the tangential direction with the gap is sinusoidal. The amount of hot gas penetrating the axial gap increases linearly with the increasing axial gap width. It is therefore important to reduce the axial gap width to a minimum allowable by tolerance limits in order to reduce the hot gas ingress.
FIG. 3 shows a prior art turbine stator vane with a cooling circuit for the lower endwall. Two adjacent vanes are shown. Cooling air flows into two inlet passages 11 in a direction toward the leading edge of the endwall, and then flows along cooling chambers 12 formed along the leading edge of the two endwalls and toward the sides where the vane endwall mate faces are formed. The cooling air then flows along a mate face cooling channel 13 toward the trailing edge of the endwalls, and then out through exit holes 15 along the trailing edge endwall. A row of cooling air exit holes 16 extend between the mate face cooling channel exit holes 15 and is supplied with cooling air from below the vane endwall. The FIG. 3 design does not prevent the hot gas ingress described above or provide adequate cooling for the endwall surfaces adjacent to the mate face that is exposed to the hot gas flow.